ON THE DEVELOPMENT OE ARANEINA. 63 



ventral face and does not extend to the dorsum of the embryo which 

 is composed of the ectoderm only. 



As to the nature of these two thickenings, the primary and 

 secondary, it is difficuU to state anything definite. The stage in which 

 the one-cell layered blastoderm is established on the surface of the 

 egg is to be looked upon as the blastosphere stage. When the ridge 

 appears in this Ijlastosphere along the line which becomes the median 

 ventral line of the future embryo and sends otf cells into the yolk 

 cavity, the whole process must be regarded as a modified form of 

 invagination and the ridge is to be looked upon as the b]asto[)ore. 

 Why there should arise two thickenings instead of one remains inex- 

 plicable to me. The primary thickening is without doubt the remnant 

 of the blastopore. Whether the secondary is to be looked upon as a 

 part of the same, I cannot decide. 



From the Formation of the Germinal Layers to the 

 Reversion of the Embryo. 



After the establishment of the ventral plate, its anterior part 

 becomes marked off as the cephalic, and its posterior part as the caudal 

 lobe, and the middle region between the two lobes is divided by trans- 

 verse rido'es into seo-ments. The least number of se^'ments observed 

 between the cephalic and caudal lobes was five. The foremost of 

 these corresponds to the segment which bears the pedipalpi and the 

 four following are the thoracic segments, each of which subsequently 

 produces a pair of ambulatory appendages. The segment which is to 

 bear the chelicerœ is soon after cut off from the cephalic lobe and the 

 abdominal segments are gradually cut off from the caudal lobe, the 

 process proceeding posteriorly, until there are formed eight abdominal 

 segments (Lycosa). 



